1. Field of the Invention
This invention relates to a continuous hot rolling method suitable for continuously hot rolling plural sheet bars inclusive of slabs at every several bars to several ten bars and a joining method aiming at rapid and simple joining operation therefor.
2. Description of Related Art
Heretofore, the sheet bar to be rolled has been heated, rough rolled and then finish rolled every one sheet bar to form a hot rolled sheet having a desired thickness in a hot rolling line. However, in such a rolling system, there are problems that the stop of the line is apt to be caused due to poor biting of rolling material in the finish rolling and the yield lowers due to the poor shape of front and back end portions of the rolling material. Recently, in order to avoid these problems and more improve the productivity, there is adopted a rolling system that the back end portion of the preceding sheet bar and the front end portion of the succeeding sheet bar are previously joined in the transfer line at an entrance side of a hot finish rolling line and then continuously fed into the hot finish rolling line.
As such a-continuous rolling of the sheet bar, there are mentioned techniques disclosed in Japanese Patent laid open No. 60-40601, Japanese patent laid open No. 60-244401, Japanese Patent laid open No. 61-159285, Japanese patent laid open No. 53-138960 and the like.
In the technique disclosed in Japanese Patent laid open No. 60-40601, the front end portion of the succeeding sheet bar is joined to the back end portion of the preceding sheet bar at an entrance of the rolling mill by pushing the cut face of the back end portion to the cut face of the front end portion. In this technique, however, it is necessary to give a certain strain to a joining region between the sheet bars in order to surely join the sheet bars at the entrance side of the finish rolling mill so as to be sufficiently durable to subsequent rolling. In order to push the succeeding sheet bar of giving such a strain, it is avoided to make the equipment itself large. Furthermore, the time required for the joining becomes long, so that when the joining is conducted at a state of stopping the transfer of the sheet bar, a long loop for absorbing it is required, while when the joining is conducted in synchronousness with the transfer of the sheet bar, the length of the equipment should undesirably be made long.
In the techniques disclosed in Japanese Patent laid open No. 60-244401 and Japanese Patent laid open No. 61-159285, the portion of the sheet bars to be joined are heated to a given temperature by the heating means and then pushed to each other to join them. According to this joining manner, the sheet bars can be joined to each other at a relatively small pushing force, but a great amount of power applied is undesirably required.
In the technique of Japanese Patent laid open No. 53-138960, it is required to take a complicated operation for making the end portion of the sheet bar into an optimum joining shape, so that it is not an effective manner.
As to the continuous rolling of the sheet bar in addition to the above techniques, Japanese Patent laid open No. 51-59749 discloses a method wherein the joining portion between the preceding sheet bar and the succeeding sheet bar is descaled or descaled and temperature-raised and thereafter the sheet bars are spot welded at a pushed state and gradually joined and rolled, while Japanese Patent laid open No. 51-130665 discloses a method wherein the back end of the preceding sheet bar is overlapped with the front end of the succeeding sheet bar and the overlapped joining portion is obliquely cut at a given angle with respect to the axial direction of the sheet bar over the widthwise direction.
Even when the descaling is carried out for joining the sheet bars by the method disclosed in Japanese Patent laid open No. 51-59749, the formation of scale reaches several ten micrometers for 10 seconds, so that it may be said that the joining strength is insufficient. In order to suppress such a scale, it is necessary to conduct the descaling or temperature rising just before the rolling, so that the length of the equipment becomes longer. Furthermore, the temperature rising is carried out in a non-oxidizing atmosphere, so that there is a problem that the equipment becomes large-scale.
In the method disclosed in the Japanese Patent laid open No. 51-130665, there is a problem that a long time is required at a step of overlapping the back end of the preceding sheet bar with the front end of the succeeding sheet bar and a step of cutting by oxygen gas or the like, and also the formation of scale is undesirably caused in the section cut by oxygen gas. Further, since the front end of the succeeding sheet bar is acceleratedly pushed toward the back end of the preceding sheet bar, the joint faces are not properly joined to each other.
As to the continuous rolling of the sheet bar, Japanese Patent laid open No. 61-144203 discloses a method wherein the back end portion of the preceding transferred sheet bar and the front end portion of the succeeding transferred sheet bar are butted over their full faces at the entrance side of the rolling mill and both side edge regions of the sheet bars in the widthwise direction are preliminarily joined and then the rolling is conducted at this state. In this method, however, an arc welding is mentioned as a concrete means for preliminarily joining the sheet bars, and such a means takes a time for prejoining, so that it is obliged to prolong the joining time for giving the sufficient joining strength at the prejoined portion and also it is required to elongate the joining line.
Further, Japanese Patent laid open No. 59-137106 proposes a method wherein a press forming machine is disposed at an entrance side of a rough rolling mill or a finish rolling mill and joining portions of the preceding and succeeding sheet bars are thinned into given shapes by this forming machine and both end portions of the bars are overlapped with each other and then such overlapped portion is pushed and spot welded.
However, the latter method has problems as mentioned below.
i) Time required for thinning the joining portions is long.
ii) When the joining is carried out at a state of stopping the press forming machine, a long loop is required, while when the joining is carried out during the running, a long running distance is required and the length of the equipment becomes long.